The physiological basis for faster growth in the Sydney rock oyster, Saccostrea commercialis

Sydney rock oysters were sampled from a mass selection experiment for growt h (the "selected" category) and from a control ("not selected") population and held in the laboratory at three ration levels. We evaluated three model s to explain faster rates of growth by selected oysters. Selection resulted in oysters feeding at up to twice the rate and with greater metabolic effi ciency than controls. A field experiment confirmed that selection leads to faster rates of feeding across a wide range of food concentrations. Selecte d oysters also grew more efficiently, at a smaller cost of growth (C-g): me an values for C-g were 0.43 J . J(-1) in selected individuals and 0.81 J . J(-1) in the controls. In contrast, oysters in both categories showed simil ar metabolic rates at maintenance, i.e., at a ration supporting zero growth . There was no evidence that differential energy allocation affected the ba lance between total metabolic requirements above and below zero net energy balance. By experimenting with selected and control oysters of different si zes and ages, then standardizing the data for size, we found no effects of age on the differences due to selection. Faster-growing oysters feed more r apidly; invest more energy per joule ingested; show a higher net growth eff iciency; and are able to allocate less energy per unit of tissue growth, th an slower-growing individuals.